Control strategies for internal combustion engines have evolved from purely electromechanical strategies to increasingly more complex electronic or computer controlled strategies. Spark-ignited internal combustion engines have traditionally used airflow as the primary control parameter, controlled by a mechanical linkage between a throttle valve and an accelerator pedal. Fuel quantity and ignition timing, originally mechanically controlled, were migrated to electronic control to improve fuel economy, emissions, and overall engine performance. Electronic airflow control systems including electronic throttle, variable cam timing, and the like, have been developed to further improve the authority of the engine controller resulting in even better engine performance.
Electronic throttle control replaces the traditional mechanical linkage between the accelerator pedal and the throttle valve with an "electronic" linkage through the engine or powertrain controller. Because of this electrical or electronic linkage, this type of strategy is often referred to as a "drive by wire" system. A sensor is used to determine the position of the accelerator pedal which is input to the controller. The controller determines the required airflow and sends a signal to a servo motor which controls the opening of the throttle valve. Control strategies which imitate the mechanical throttle system by controlling the opening of the throttle valve based primarily on the position of the accelerator pedal position are often referred to as pedal follower systems. However, the ability of the controller to adjust the throttle valve position independently of the accelerator pedal position offers a number of potential advantages in terms of emissions, fuel economy, and overall performance.
The driver controls output of the engine or powertrain primarily based on the position of the accelerator pedal. The engine control strategy must interpret this driver demand and set the appropriate engine control parameters to provide a corresponding powertrain output. The driver demand may be interpreted as a request for a particular throttle angle (pedal follower system), an engine torque, a wheel torque, or power (torque*speed). To achieve a consistent driver "feel", many prior art control strategies attempt to provide a constant powertrain output for a particular accelerator pedal position, regardless of the current operating conditions, such as barometric pressure. While these strategies consider the current operating conditions, such as barometric pressure, in determining the control parameters for the engine, the operating conditions are used to maintain a constant output independent of the operating conditions. However, this strategy may result in unsatisfactory performance when the available engine power is reduced, such as when driving at high altitude (low barometric pressure).